The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as Nell as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
When an internal combustion engine is initially started, the engine and an exhaust system of the engine are cold. Further, an intake manifold absolute pressure (MAP) is high (i.e., near atmospheric pressure) because the engine is initially turning at a very low rate. The combination of low temperature and high pressure make it difficult for fuel to vaporize in the cylinders. Incompletely vaporized fuel is not completely combusted. A portion of fuel that is riot completely combusted at start-up causes increased hydrocarbon (HC) and carbon monoxide (CO) emissions.
Vehicles that include an internal combustion engine may also include a catalytic converter to treat exhaust. The flow of exhaust from the engine to the catalytic converter may be called an exhaust stream. The catalytic converter includes a catalyst that catalyzes the oxidation of HC and CO as well as the reduction of nitrogen oxides in the exhaust stream. Typically, the catalyst does not efficiently control emissions at start-up because: 1) a higher fraction of fuel remains unburned and is subsequently exhausted, and 2) the catalyst is not sufficiently hot.
The catalyst operates efficiently when the catalyst reaches an operating temperature called a light-off temperature. The catalyst reaches the light-off temperature after the engine and the exhaust system have been adequately heated from engine operation. As time elapses after the engine is started, the engine speed increases, the MAP decreases, and the operating temperature of the catalyst increases. The combination of low MAP and increased operating temperature allows the fuel to vaporize adequately, thereby resulting in more complete combustion of the fuel.